Why does my aircon leak only when it rains?
A leak that only shows up when it rains feels like a drain fault, but it rarely is. Rainwater can enter through a wall opening, weather pressure can disrupt drainage, or moisture can reach electrical points. Three different faults look identical from the puddle alone.
By Team Snowflake | Reviewed 30 May 2026
1. External rain ingress through wall or trunking path
Clean water appears along a wall seam, trunking edge, or indoor frame only when it rains. The exterior wall hole for pipes and cable may have failed sealant. Rain enters the sleeve, runs along the pipe route, and appears at the lowest visible point.
How to tell
Unlike drain backflow, which needs the unit running plus wind pressure, rain ingress leaks whether the unit is on or off. It tracks directly with rainfall and repeats on every rain event, not just the worst storms. Trace the water trail back toward the wall opening. If the sealant there is intact and dry, this path is ruled out.
- Water appears during rain and dries up once the weather clears.
- The drip trail follows the trunking run or a wall seam.
- Cooling still feels normal and the drip continues with the unit off.
How we confirm it
We trace the entry path from outside. We reseal the wall penetration and trunking gap with waterproof compound. Then we confirm the indoor drip stops after the next rain.
Avoid gas top-up or coil cleaning before the wall penetration and rain path are checked. The leak returns with the next storm.
2. Drain path affected by weather pressure pattern
You notice the drip only when the unit is running during heavy rain with strong wind, and it stops once you switch off. The condensate drain exits to the outside, exposed to the weather. During a Singapore squall, sustained wind pressure builds at the outlet. It pushes air, and sometimes water, back up the pipe until the drain tray overflows.
How to tell
Unlike rain ingress through the wall, this backflow needs the unit running during heavy rain and strong wind together. The key test: does the drip stop the moment you switch the unit off mid-storm? If it does, weather pressure is your pattern. Unlike the electrical path, there is no breaker trip and no burning smell with it.
- The drip shows up only while the unit runs during windy rain.
- The water pattern shifts with wind and rain intensity.
- Switching the unit off during a storm stops the drip quickly.
How we confirm it
We inspect the drain outlet exposure and routing. If back-pressure is confirmed, we fit a wind guard or reposition the discharge point. Then we retest while the unit runs.
Jumping to a drain flush or high-pressure clearing treats a blockage that is not there. It masks the weather-pressure cause, and the leak comes back every squall season until the outlet is guarded or moved.
3. Rainwater near electrical points
Rainwater can enter at the wall penetration, then travel inside trunking or wiring conduit before it appears. If it reaches the isolator, terminal block, or PCB wiring, it becomes a shock and fire risk. This is the rain-leak path to rule out first.
How to tell
This path has electrical signs. Unlike wall ingress or drain backflow, the issue is not only cosmetic water damage. Breaker trips, burning smell, plastic odor, or moisture near an isolator show rainwater has reached live parts.
- Water appears near the switch, isolator, or wiring points.
- The breaker trips during a rain-linked leak.
- An electrical or plastic smell shows up with the drip.
How we confirm it
Stop using the unit until electrical safety is confirmed. We isolate the ingress, then dry and inspect the live points. We verify a safe power condition before any restart.
Stop using the unit and turn the circuit breaker off while water is near electrical points. Do not run repeated test restarts to see if it trips again. That stresses the components and can turn a small fault into a costly board or motor failure.
Useful next steps
Related reading
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